perf(agent): unranked BM25 scores and a top-k merge — same rankings, 4-5x faster

Fusion min-max normalises over every keyword match, so hybrid_search asked
BM25 for a ranked list of the whole corpus: a hash insert per posting, then a
sort of every match, then the merge sorted every candidate again to keep k.

- BM25Index::scores returns every match unsorted, accumulated in a dense array
  (contributions are strictly positive, so zero means untouched). search() is
  built on it with the bounded heap.
- merge_vector_keyword partitions out its top k (select_nth) and orders only
  those, with the same score-then-id order.
- Both hybrid paths use scores().

Rankings are identical (equivalence tests for both changes). p50 0.24 -> 0.07
ms (1K), 2.1 -> 0.49 ms (10K), 23 -> 4.65 ms (100K).

The harness gains --fusion-study, which measured the alternative — capping the
keyword pool — and found it changes the top-10 for most queries (overlap
0.83-0.92, different #1 for 10-35%) for only a 2x saving. Not adopted.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

CHANGELOG.md

@@ -43,6 +43,14 @@
   on every kernel call.
 - `clawhdf5-ann`: `HnswIndex::graph_to_bytes` / `from_graph_bytes` — graph-only
   serialization (checksummed, every neighbour id and level validated on load).
+- `clawhdf5-agent`: a further 4-5x on `hybrid_search` with **identical
+  rankings** (p50 now 0.07 / 0.49 / 4.65 ms at 1K / 10K / 100K — 79x / 100x /
+  190x faster than v2.3.0). Fusion needs every keyword score but not their
+  ranking: new `BM25Index::scores` returns them unsorted from a dense
+  accumulator (it hashed every posting, then sorted every match), and
+  `merge_vector_keyword` selects its top k instead of sorting every candidate.
+  Capping the keyword candidate pool was measured and rejected: it changes the
+  top-10 for most queries (`search_harness --fusion-study`).
 - `clawhdf5-agent`: BM25 results are deterministic (ties break by record id),
   top-k uses a bounded heap, and the "WAND early termination" that computed a
   bound and then ignored it is gone. IDF is computed per query.